This proposal aims to contribute to an understanding of the organization of elementary genetic units in higher organisms and the regulation of the expression of such units in development. The rosy locus of chromosome 3 in Drosophila melanogaster carries the structural information for a peptide which, as a homodimer, functions as xanthine dehydrogenase in this organism. Mapping experiments have served to map the borders of the structural element and variants which map outside and just to the left of the structural element appear to be associated with regulation (i.e., they are alterations in a cis acting control element). The direction of experiments in the current proposal are: (1) mutagenesis experiments designed to make several classes of control variants, (2) analysis of such variants, (3) examination for polygenic clustering about the rosy locus, (4) developmental characterization of control variants, and (5) analysis of EMS mutants for nonsense or chain terminating mutants in order to determine direction of transcription and translation. BIBLIOGRAPHIC REFERENCES: Gelbart, W., M. McCarron and A. Chovnick. 1976. Extension of the limits of the XDH structural element in Drosophila melanogaster. Genetics 84:211-232. Chovnick, A., W. Gelbart, M. McCarron, B. Osmond, E.P.M. Candido and D.L. Baillie. 1976. Organization of the rosy locus in Drosophila melanogaster: Evidence for a control element adjacent to the xanthine dehydrogenase structural element. Genetics 84:233-255.